1. Field of the Invention
The present invention relates to an ivory-colored zirconia sintered body, a process for the production thereof and use thereof as a bracket for orthodontic application.
2. Prior Art
Due to high strength and excellent gloss of a mirror-polished surface, a zirconia sintered body, particularly a tetragonal zirconia sintered body, has been increasingly applied to the production of home utensils such as a knife, sports goods such as golf shoes and spikes or track shoes, while it has been also begining to be applied to the production of seals, accessories, frames and faces of watches and frames of ophthalmic lenses as a substitute for an ivory. Zirconias in a variety of colors are demanded in order to meet with these expanding requirements. In particular, a substitute for an ivory is desired to have a color closer to the natural color of an ivory. However, there is no report on a zirconia having such a color.
Meanwhile, stainless steel has been and is used as a material for a bracket for orthodontic application. That is because a bracket attached to each tooth is forcibly pulled with a wire made of a metal, etc., for orthodontic application and is therefore required to have high strength and high toughness so as to be free from snapping and chipping, and stainless steel meets with these requirements. However, stainless steel involves a defect in that it has metal gloss and shows its color, or is inferior when aesthetically appreciated. To overcome this defect, JP-A-64-25847 discloses a bracket for orthodontic application, which is formed from single crystal alumina. JP-A-64-46451 discloses a bracket for orthodontic application, which is formed from single crystal zirconia containing yttria. Further, JP-A-64-52448 discloses a bracket for orthodontic application, which is formed from spinel type crystalline ceramics obtained from magnesium oxide and aluminum oxide. Furthermore, JP-A-2-21857 discloses a bracket for orthodontic application, which is formed from a material prepared by incorporating iron oxide, manganese oxide, nickel oxide or a mixture of these into zirconia partially stabilized with yttria, etc.
The single crystal alumina described in the above JP-A-64-25847 is excellent in transparency and hence aesthetically satisfactory. However, this single crystal alumina has the following defects. Its processing requires a high cost. Since it has directional strength and low fracture toughness, it is liable to be snapped or chipped, and its broken surface forms a sharp edge like broken glass.
The single crystal zirconia containing yttria, described in the above JP-A-64-46451, and the spinel type crystalline ceramic described in the JP-A-64-52448 are transparent and aesthetically excellent. However, these materials have a defect in that they have low mechanical strength and are therefore liable to snap.
The partially stabilized zirconia containing a transition metal oxide such as iron oxide, etc., described in JP-A-2-21857 may exhibit a variety of colors. However, an ivory color cannot be obtained. And, when this partially stabilized zirconia is attached to teeth, it cannot be said to be aesthetically satisfactory, since it is poor in translucency.